In a toroidal type continuously variable transmission for a vehicle as for example disclosed by U.S. Pat. No. 5,542,890, a speed change ratio varies according to a gyration angle of a plurality of power rollers enclosed between input discs and output discs. These power rollers are each supported by a trunnion.
The gyration angle of the power roller varies according to a force acting on the trunnion in a specific direction at right angles to both the rotation axis of the power roller and the rotation axis of the discs. This variation depends on a load applied by the input disc and output disc on the power roller according to a displacement of the power roller in the aforementioned specific direction.
Such dynamics are disclosed or example by SAE Technical Paper No. 901761.
The trunnion is driven by exerting an oil pressure supplied from a speed control valve on a piston which is fixed to the trunnion, as indicated for example in Tokkai Hei 3-89066 published in 1991 by the Japanese Patent Office. The speed control valve varies the oil pressure in response to a step motor which rotates according to a target speed change ratio. The target speed change ratio may be determined for example according to a vehicle speed Vsp and a throttle opening TVO.
On the other hand, the displacement of the trunnion is fed back by the speed control valve through a precess cam linked to the trunnion. In this way, feedback control of the power roller gyration angle is performed so that a real speed change ratio coincides with the target speed change ratio.
In this continuously variable transmission, in order to achieve the dual objectives of strength and lightweightness, the transmission casing and the housing of the speed control valve are constructed of an aluminum alloy, whereas parts which transmit mechanical force such as the trunnion, piston and precess cam are constructed of an iron alloy.
When the temperature of the parts and working oil rises according to running conditions, the trunnion which is constructed for example of iron alloy expands in the direction of displacement, while at the same time, the transmission casing which is constructed of aluminum alloy expands to a greater degree due to the difference of thermal expansion coefficient of iron and aluminum.
The neutral position of the continuously variable transmission is the position where the rotation axis of the power roller intersects with the rotation axis of the input/output discs in one plane. However, when the thermal expansion coefficients of the mechanical parts are different, this neutral position varies with temperature, and it becomes difficult to realize a precise target speed change ratio when the temperature changes. According to studies carried out by the inventors, a shift of 9% or more occurs in the speed change ratio for a temperature difference of 50.degree. C.